A healthy luminal endothelium consists of quiescent endothelial cells that provide a regularly organized and relatively non-adhesive surface towards the blood constituents. However, this interface is dynamic, and the endothelial cell phenotype can undergo profound changes leading to an activated phenotype. The activated phenotype represents a time- and dose-integrated response to various stimuli originating from the circulating blood and/or from neighboring cells and tissues. Among these stimuli, inflammatory cytokines (IC) such as interleukin-1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ), or angiogenic growth factors (AGF) such as vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF) have been shown to activate endothelial cells by altering their pattern of gene expression and thereby, altering their properties and phenotype. With respect to the biological activities of endothelial cells two different activated phenotypes can be distinguished (FIG. 1). On the one hand, there is the adhesion-competent and non-proliferative phenotype, which can be induced by the IC. The other quite opposite phenotype is characterized by increased motility and cell proliferation, which is induced by AGF. In an in vivo situation all cytokines and growth factors potentially activate the endothelial cells. Consequently, the phenotype of endothelial cells will depend on their ability to respond to the local balance of inflammatory cytokines and growth factor concentrations.
The IC such as IL-1β, TNF-α or IFN-γ are mainly secreted by activated lymphocytes, monocytes or macrophages. Beside many other stimulating or regulatory effects on various cell types one common feature of said IC is the upregulation of the expression of adhesion molecules—such as ICAM-1, VCAM-1, or ELAM-1—on the endothelial cell surface, which leads to an adhesion competent state of the cells. This adhesion competent state is defined as the period where a cell is competent to adhere to other cells or to an extracellular matrix. A well-known example is the increased adhesion of activated leukocytes to the endothelium. Additionally, it is described that IL-1β, IFN-γ and TNF-α exert an anti-proliferative effect on endothelial cells. Moreover, IFN-γ and TNF-α have even been shown to induce apoptosis in endothelial cells in vitro.
By contrast the AGFs, such as VEGF and bFGF, function as potent mitogens and chemotactic factors for endothelial cells. bFGF is a pleiotropic factor that has been found in almost all normal tissues and has a broad specificity for a number of target cells. Moreover, bFGF was shown to induce angiogenesis in vitro and in vivo. VEGF is a secreted protein that shows a high specificity for endothelial cells and promotes angiogenesis during normal physiological processes. Furthermore, VEGF strongly induces microvascular permeability that is also a general feature of tumor-associated microvasculature. Several lines of evidences suggest that VEGF and bFGF which display synergistic effects in the induction of angiogenesis in vitro and in a mouse model in vivo might contribute to the pathological process of tumor-associated angiogenesis (Plate et al., 1992, Nature 359, 845-848).
Considering that activation of endothelial cells plays a critical role in many physiological adaptations or pathological dysfunctions such as inflammation, haemostasis, immune reactions, psoriasis, atheriosclerosis, wound healing, angiogenesis or neovascularisation e.g. of malignant tumors (Cotran & Pober, 1988, Endothelial activation: its role in inflammatory and immune reactions. In endothelial cell biology, N. Simionescu and M. Simionescu, eds. (New York: Plenum Press), pp. 335-347) there is a high need to understand, influence and control the different stages of activation of endothelial cells. Additionally, a clear determination of the different phenotypes of endothelial cells is of particular interest for a decision on a proper therapeutic strategy, which can provide relief for a patient suffering from the medical syndromes adjacent to the activation of endothelial cells as mentioned above.